Phenology in the Forest

By Bruce Rottink, Volunteer Nature Guide and Retired Research Forester

In the forest, like much of life, timing is everything!  It’s why most animals have their young in the spring or early summer when food is abundant.  It’s why most plants don’t bloom in December, when there’s a good chance that their flowers would be killed by a subsequent frost.  The study of the timing of different biological events is called “phenology.”


What’s a Phenology Study?

A phenology study involves identifying when different organisms enter different stages of their life, or behave in particular ways.  My phenology study focuses largely on plants.  Plant phenology frequently involves studying the behavior of selected individuals over the course of several years.  Some “events” in a plant’s life that can easily be tracked are, for example, when the veins on the new leaves are first visible (aka, bud break), or the first time you can see the sex organs inside of a flower.

I started the phenology study at Tryon Creek State Natural Area (TCSNA) early in 2013.  For this study I tracked ten different species growing along 4 different trails at the park; Red Fox, Old Main, Cedar/West Horse Loop (referenced here as “Cedar”) and Middle Creek/Big Fir (referenced here as “Middle Creek”).  For perennial species with above ground parts, I tagged the plants and followed them each year.  For annual plants, or those species arising from underground organs, I identified a given patch of ground and studied plants at that location.  As time went by, I started including observations on a few other species like a delightful patch of bleeding hearts (Dicentra formosa), and some spittlebug nymphs (suborder: Auchenorrhyncha).

I made observations on a weekly basis, with a few exceptions caused by vacations, and extreme weather conditions.  For a couple of reasons, mostly related to “learning curve” issues, I starting collecting useable data in 2013 part way through the growing season.

One of the challenges in conducting a phenology study is the issue of when you should report the results.  In this case, the differences I observed between 2016 and 2017 are dramatic enough that it is time to provide you with a report.  This probably won’t be my last phenology report, “God willing and the creek don’t rise” (to use an old expression).


The Drivers of Plant Development

Plant development is driven by several factors, key among them being day-length, temperature and moisture availability.  When it comes to spring budburst in our area of the world, temperatures probably are the primary driver.

The temperature plays two important, and quite different, roles in bud break.  The plant needs to hold off on bud break until the threat of a killing frost is past.  Thus most perennial plants in our area have a “chilling requirement.”  This means that the buds have to experience a certain amount of chilling before they can start growing.  Secondly, the buds have a “forcing requirement” which is a certain amount of warm temperatures to get the buds growing after the chilling requirement has been met.  As anyone who has ever walked through the forest in the spring knows, these requirements vary dramatically between different species of plants.  If the plants receive less than the normal amount of chilling curing the winter, they will need a greater amount of warm “forcing” in the spring.  Although it is clear from the diagram that there is at least some minimal amount of chilling needed to ensure that the buds will eventually open.

The diagram below shows the generalized nature of the relationship of chilling and forcing for both Douglas-fir (Pseudotsuga menziesii) and western hemlock (Tsuga heterophylla).  While the curves have a basically similar shape, it is apparent that with low levels of chilling, the hemlock will break bud first, but with large amounts of chilling, the Douglas-fir will break bud first.

Chilling and forcing requirements of Douglas-fir and western hemlock1photo 1

            Photo 2


In reviewing these bud break results, please be aware that there is very little agreement on the exact temperature that separates the “chilling” and “forcing” functions.  I believe most scientists would think 50 degrees is little bit too high, but to be blunt, this is the base I used because it is the best database to I have available.  As you can see in the chart below, there have been dramatic differences between years in the number of growing degree days in the first three months of the year, primarily that 2017 has a much cooler spring.


Photo 3


photo 4

*Data from the Aurora Airport, approximately 15 miles from TCSNA.


So What Happened?

Presenting even a summary of all the data I collected would be a sure cure for insomnia, so I’ve picked out a couple of examples from the study which are fairly typical of the general trends.  The first example is to look at the behavior of the Pacific waterleaf (Hydrophyllum tenuipes).  This is a plant that has underground roots, stems and buds.

The graph below shows the results for the years 2015 through 2017.  On average, the appearance of the first leaves in spring 2017 was delayed an average of 2.5 weeks from the first appearance of leaves in the prior two years.  (And yes, the absence of data for the week of Feb 11, 2015 is unfortunate!)  The date of first flowering in 2017 was on average 3.0 weeks later than first flowering in 2016.   The primary lesson here is that both budburst and flowering in 2017 was much delayed compared to the two prior years.  Interestingly, the average date of when the last leaves died was nearly identical in 2016 (33.0 weeks) and 2017 (33.75 weeks).


Photo 5

Development of Pacific waterleaf on four trails between 2015 and 2017


Similarly, the leafing out of the vine maple (Acer circinatum) was also delayed about 3 weeks in 2017, as seen in the chart below.  For the vine maple, so few of the plants I followed produced flowers on a regular basis that the data is probably not worth presenting, although what data there is follows the same general pattern as the Indian plum above.


Photo 6

Leafing Out and Leaf Retention Times for Vine Maple


photo 7

Growing Degree Days (50o degree base) for 4 Years


At first glance, these lines all look fairly similar.  However, looking at Week 20, for example, the number of growing degree days in 2016 is about double the number for that same date in 2017.  This is a huge difference!

The final set of plant data that I will include here is for snowberry (Symphoricarpos albus), a reasonably common, but not abundant shrub at TCSNA.  Here again, both the budburst and flowering of these plants is three to five weeks later in 2017 compared to 2016, as seen in the graphs below:


Photo 8


And it’s not just plants

 Below is a chart of the sightings for two years of spittlebugs.  In the beginning of 2016, if there were no spittlebugs seen, I just left the space on my datasheet blank.  Midway through that year, I recognized the folly of that approach, and started making a clear record showing that no spittlebugs were seen.  My notes on the March 2016 spittlebug indicate that it was just one individual bug, and a small one at that.  Sometimes Mother Nature will show off one specimen of something way out of season, but that probably doesn’t really mean the season has started.  No matter what you think about the March 2016 outlier, it is abundantly clear that the spittle bug, like many of its botanical associates, was late in 2017 by at least three weeks.


Photo 9


The Connection to Global Warming

As documented here in the differences between the various years, the organisms in the forest are sensitive to environmental temperatures.  This will be very important as we consider global warming, and what we should do about it.  I love the fact that Mother Nature provided us with a great example of the fact that the trend of global warming is not a straight linear process, but will have some hiccups like 2017.  By the way, the early readings from this year (2018) show that some plants are starting growth way earlier than they did in 2017.  But that’s grist for another note at least a year away.  A fascinating aspect to this, which we may be many years away from experiencing, is that almost all woody plants require a certain amount of chilling before they break bud.  If global warming ever gets to the point where the winter temperatures are not adequate to chill the buds, already completed research tells us bud break will be significantly delayed.  Then we could have real problems.  But for now, enjoy the forest that we have!


1Harrington, Connie and Peter Gould.  2016.  Rise and Shine: How Do Northwest Trees Know

When Winter Is Over?  Science Findings, Issue 183.  USDA Pacific Northwest Research Station.


“When Autumn Leaves Start to Fall”…or Not!

By Bruce Rottink, Volunteer Nature Guide and Retired Research Forester

It’s a sure sign of autumn at Tryon Creek State Natural Area (TCSNA) when the leaves start to fall.  However, the time of leaf fall can vary dramatically between species.  I became aware of this during the past 5 years of my phenology study.  This phenology study involves visiting more than 60 specific selected plants at TCSNA every week to ten days throughout the year.  I make notes on the stage of development of each plant on each visit.  For example, I note stages of development like “buds open enough to see leaf veins”, “flower’s stamens visible” or (in the autumn) “last leaf has fallen.”  This year, for example, every one of the vine maples (Acer circinatum) in the study had lost all their leaves by November 22nd.  In contrast, several thimbleberries (Rubus parviflorus) still had numerous green leaves as late as December 13th.  This is a difference in leaf fall between species of more than 3 weeks.  Sometimes the difference in the date of leaf fall varies dramatically even on the same plant.

This year I found numerous examples of extra-long leaf retention in several plants.  These plants appear to have differences in time of leaf fall for one of two different reasons.


Adventitious Buds

Adventitious buds are buds that form from normal plant tissues.  Oftentimes they arise from the cambium, the layer of soft, actively dividing cells just under the bark of woody plants.  Normally the cambium cells just differentiate into either bark or woody stem cells.  Most plants, like Douglas-fir (Pseudotsuga menziesii) and bigleaf maple (Acer macrophyllum), recognize that the energy they need for growth is coming from the sun up above.  Their secret to success is growing tall, where they can reach above most plants, and bask in the full sunlight.  To do this, these plants want to direct as much energy as possible into the shoots that are growing straight up towards the sun.  The apices of these plants “control” the growth of side buds and shoots by producing a chemical named auxin.  The chemical structure of auxin is shown below.


Photo 1

The chemical structure of auxin


This auxin is produced at the tip of the dominant stems.  As the auxin is transmitted down the stem, the message that this chemical delivers to the lower buds is “don’t grow” and “don’t develop new buds” depending upon the exact circumstances.  When the top of the tree, or the tip of a branch, is killed or broken off, the auxin no longer flows down the stems, and eager lower buds start to grow out.  In some cases, brand new buds are formed along the stem, and take off like a rocket!

One of my favorite examples of this at TCSNA is found on a Pacific yew (Taxus brevifolia) tree along the Cedar Trail.  The tree blew over during a storm and the trunk lay across the trail, although some roots were still in the ground.  On January 22, 2017 I cut off the trunk to allow the trail to be more accessible.  But the big news was months later.  All along the remaining trunk, brand new little yew branches were growing out, no longer inhibited by the top of the tree, as shown in the picture below.


Photo 2

Me sawing off the top of a yew tree which feel across Cedar Trail. Note the trunk is bare of sprouts.


Photo 3

Same yew trunk on August 10th showing new sprouts


Another example started last spring when a trail maintenance crew cut the top off a lot of shrubs along the Red Fox trail.  This produced results similar to what happened on the Pacific yew.  One of the plants they cut back was the hazel (Corylus spp.) shown below.  The black arrow is pointing to a stem which has lost all of its leaves, and in fact, almost all of the plant has lost its leaves.    In contrast, the purple arrow points to one of the two stems on the plant which is still holding onto its leaves.


Photo 4

Hazel (Corylus spp.) bush on November 29, 2017


So what’s the difference?  Note the blue and red circles in the picture.  The blue circle shows where a major stem of the plant was cut off by the trail maintenance crew between May 30 and June 6.  On this particular plant, the leaves had started growing about March 8 and were approximately 2 inches long at the time the bush was trimmed back.  The removal of this main stem liberated buds on the lower parts of the stem.  These liberated buds began to grow (note purple arrow and red circle).  So these adventitious shoots and their leaves were much younger than the shoots and leaves of the other parts of the plant.  These young leaves stayed on the plant much longer than the “normal” leaves.

Two other examples of this phenomena are shown below.  The first is a red huckleberry (Vaccinium parvifolium) which was also pruned off (note red arrow).         


Photo 5

Adventitious shoots on a red huckleberry near Red Fox Trail.




Photo 6

Normal shoots on a red huckleberry near Red Fox Trail.




The final example is an Indian plum (Oemleria cerasiformis) plant.  Here the branch tip appears to have been randomly broken off, rather than cut off, but the end result is the same.  All the leaves on this plant, except some leaves on the small shoot shown in the left picture had fallen off by September 28th.  The fact that this one leaf is still green and healthy 10 weeks after all the “normal” leaves had fallen off the plant is astounding.



Photo 7

Leaf on an adventitious shoot on December 13th on an Indian plum growing near Cedar Trail.


Photo 8

Leaf on a normal shoot on December 6th on the same Indian plum as above, growing near Cedar Trail.



Buds:  Preformed or Neoformed?

The second reason that some leaves might hang on longer than others is due to when the leaves are formed.  For example, in overwintering Douglas-fir buds, each of the needles that will appear the following growing season is already formed.  You can see these “baby needles” as little whitish bumps (red arrow) in the picture below after all the bud scales (blue arrows) have been cut off.



Photo 9

Dormant Douglas-fir bud after removing the protective scales.



In contrast to these “pre-formed” buds, some plants have a different (“neoformed”) growth strategy.  In these plants, new leaves (“primordia”) will be initiated during the spring and summer as long as growing conditions are favorable.  These primordia will immediately start to develop into leaves.  Thus on one plant, there will be leaves of vastly different ages.  Not surprisingly, the older leaves will drop off before the younger ones.  One example of this at TCSNA is the red elderberry (Sambucus acemose).  I took the pictures below of one plant on December 6, 2017 at TCSNA.



Photo 10

Buds. but no leaves on lower branches on a red elderberry along Cedar Trail.


Photo 11

Healthy leaves on the upper branches of a red elderberry along Cedar Trail.



This continual development of new leaves is illustrated in the chart below of the length of 3 different leaves throughout the growing season, until somehow the plant’s stem was severed.


Photo 12

Leaf length of the 7th, 8th, and 9th leaves on a red elderberry stem.



Photo 13

Tip of red elderberry stem (in winter) which is dead beyond the pair of red-brown buds



The elderberries’ strategy is successful although when the end of the growing season comes, the youngest segment of the twig hasn’t had a chance to produce viable buds, and the twig beyond the last pair of viable buds just dies.


There’s more than one road to success

Mother Nature has produced many species of plants, which use a variety of strategies to achieve success.  Two of the strategies involve leaf and shoot development.  As illustrated above, the first strategy involves focusing the plant’s energy on the development of just a few leading shoots and suppressing the others.  However, Mother Nature knows that bad things happen, and if those leading shoots are killed or injured, tissue lower down on the plant can create branches that will take over and keep the plant alive.  The second strategy involves having a stem that just keeps elongating and producing new leaves as long as the weather is good.  Both strategies have been successful, and add more interest to our forest.

 –All photos by Bruce Rottink

Recycling the Forest: Year 3

By Bruce Rottink, Volunteer Nature Guide and Retired Research Forester


The plants growing at Tryon Creek State Natural Area (TCSNA) require a supply of nutrients to stay healthy and keep growing.  An important part of supplying those nutrients is the decay of dead organic matter like leaves and branches.  This decay process releases the chemicals in the dead leaves and branches so those nutrients can be reused by plants that are still growing.


Study Procedure

To study this process, I collected leaves, branches or cone scales of several species of trees found at TCSNA in September 2014.  I only collected fresh materials which had recently fallen to the ground.  Within two days I placed these in wire mesh (screen) “envelopes” and fastened the envelopes to the ground with nails.  During the first part of the study I took photographs of the envelopes on a monthly or quarterly basis, but starting in September of 2016 I switched to once-a-year monitoring.  The results after one and two years have already been published in earlier Naturalist Notes.

The one-, two- and three-year end results are presented here.  While I had two envelopes for each of the materials, I only selected the most photogenic envelope for inclusion in this report.


Red Alder (Alnus rubra) Leaves

Red alder is a common tree at TCSNA, and is renowned for dropping its high nitrogen leaves onto the ground while they are still green.  The results from this study are seen in the photos below.

Photo 1

Left: Start (0 years). Right: After 1 year.

Photo 2

Left: After 2 years. Right: After 3 years.


One university research project studying the decay of dead red alder leaves indicated that 93% of the nitrogen in the alder leaves was released into the soil via the decay process the first year.  The same study found that 91% of the calcium and 97% of the potassium (both key nutrients) were also released during the first year of leaf decay1.  This relatively rapid decay of the red alder leaves was attributed to the fact that they generally contain higher levels of nutrients than most trees, thus nourishing the decay organisms.


Western Redcedar (Thuja plicata) Branchlets

Photo 3

Left: Start (0 years). Right: After 1 year.


Photo 4

Left: After 3 years. Right: After 3 years.

In sharp contrast to the red alder leaves, the western redcedar foliage contains lower levels of many nutrients, thus making it a somewhat less attractive food source for microorganisms.  In addition, redcedar foliage and twigs contain many terpenes.  Terpenes not only give the tree it’s distinctive “cedar smell,” but terpenes also have anti-fungal properties.  The major terpene in western redcedar (α-thujone ) is shown below.


Photo 5.jpg

Chemical structure of α-thujonecaption


No wonder that the cedar is decaying a little more slowly than some of the other samples.


Bigleaf Maple (Acer macrophyllum) Leaves

Photo 6

Left: Start (0 years). Right: After 1 year.


Photo 7

Left: After 2 years. Right: After 3 years.


With no special chemical defenses against fungi, and a fairly high nutrient content, the maple leaves are another fast decaying material.


Douglas-fir (Pseudotsuga menziesii) cone scales

These Douglas-fir cone scales were already lying on the ground at the time of collection.  The cones they came from had probably been chewed apart by a squirrel looking for the nutritious seeds.


Photo 8

Left: Start (0 years). Right: After 1 year.

Photo 9

Left: After 2 years. Right: After 3 years.



Superficially, the cone scales after three years look almost exactly like they did the day I put them in the bag.  Obviously the scales are made up of a very hard material, not too much different than the wood in the main stem of the tree.  Mother Nature made the cone scales very durable to protect the precious young seeds, and as a result, they are being recycled very slowly.


Douglas-fir branchlet

Photo 10

Left: Start ( 0 years). Right: After 1 year.

Photo 11

Left: After 2 years. Right: After 3 years.


Again, the Douglas-fir twigs and foliage contain some terpenes similar to the terpenes found in the redcedar, and thus, they decay more slowly than the alder and maple leaves.  Notwithstanding the resistance to decay, all the needles have fallen off the twigs.


The End Result

The nutrients within each of the samples used in this study will be released and then absorbed by living plants and fungus, helping the forest to grow the next generation of life.  This study shows the enormous differences in the decay times of different kinds of tree litter.  The softer materials, like the alder and maple leaves also have the highest nutrient content, and lowest concentration of anti-fungal chemicals.  But, as the old saying goes, “All in good time!”  All these nutrients will once again join living organisms, ensuring the continuation of TCSNA’s forest!


1Radwan, M. A., Constance A. Harrington and J. M. Kraft.  1984.  Litterfall and nutrient returns in red alder stands in western Washington.  Plant and Soil 79(3):343-351.


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